Ion chromatography and other forms of liquid chromatography are widely used analytical techniques for determination of ionic analytes. Dilute solutions of acids, bases, and salts such as sodium carbonate and sodium bicarbonate are used as eluents in the ion chromatographic separations. Traditionally, these eluents are prepared off-line by dilution with reagent-grade chemicals. Off-line preparation of chromatographic eluents can be tedious and prone to operator errors, and often introduces contaminants. For example, dilute NaOH solutions, widely used as eluents in the ion chromatographic separation of anions, are easily contaminated by carbonate. The preparation of carbonate-free NaOH eluents is difficult because carbonate can be introduced as an impurity from the reagents or by adsorption of carbon dioxide from air. The presence of carbonate in NaOH eluents often compromises the performance of an ion chromatographic method, and can cause an undesirable chromatographic baseline drift during the hydroxide gradient and even irreproducible retention times of target analytes. Therefore, there is a general need for convenient sources of high purity acid, base, or salt for use as eluents in the ion chromatographic separations.
U.S. Pat. No. 5,045,204 describes an impure acid or base is purified in an eluent generator while flowing through a source channel along a permselective ion exchange membrane which separates the source channel from a product channel. The membrane allows selective passage of cations or anions. An electrical potential is applied between the source channel and the product channel so that the anions or cations of the acid or base pass from the former to the latter to generate therein a base or acid with electrolytically generated hydroxide ions or hydronium ions, respectively. This system requires an aqueous stream of acid or base as a starting source or reservoir.
U.S. Pat. Nos. 6,036,921 and 6,225,129 describe electrolytic devices that can be used to generate high purity acid and base solutions by using water as the carrier. Using these devices, high purity, contaminant-free acid or base solutions are automatically generated on-line for use as eluents in chromatographic separations. These devices simplify gradient separations that can now be performed using electrical current gradients with minimal delay instead of using a conventional mechanical gradient pump.
Dilute solutions of salts such as sodium carbonate and sodium bicarbonate are often used as eluents in ion chromatographic separations. One object of the present invention is to develop methods and devices for generating such high purity salt solutions using water as a carrier.